Conventional hot melt adhesives are 100% solid, solvent free adhesives that, in general, can be repeatedly heated from their solid state to form a liquid state. Although reactive hot melt adhesives are also 100% solid and solvent free, they behave more like thermoset adhesives and go through irreversible chemical reactions once dispensed and in the presence of ambient moisture.
Polyurethane reactive hot melt adhesives are disclosed in U.S. Pat. Nos. 3,931,077; 4,775,719; 4,808,255; 4,820,369; 4,891,269; 5,021,507 and 5,441,808; European Patent Application Nos. 0 532 765 A1 and 0 632077 A2. The use of catalysts is also disclosed in some of the above mentioned cases. In general, catalyst levels of 0.2% and above are used for improving the cure rate.
U.S. Pat. No. 5,550,191 discloses a catalyzed polyurethane reactive hot melt adhesive having: a) a reactive polyurethane adhesive; and b) an effective amount, generally from 0.05% to 0.6% by weight of a 2,2'-dimorpholinoethyl ether or di(2,6-dimethylmorpholinoethyl) ether catalyst. The invention also discloses a process for improving the curing speed of a reactive polyurethane hot melt adhesive by the incorporation of the same ether and morpholine containing catalyst. It is disclosed that the curing speed of polyurethane reactive hot melt adhesives may be significantly improved by the use of a catalyst containing both ether and morpholine functional groups at these generally low levels of 0.05% to 0.6%, but preferably at levels of 0.075% to 0.125%, and most preferably at levels of about 0.1%. It is also disclosed that this can be accomplished with little or no effect on thermal stability.
Surprisingly, the present inventors have now discovered that even lower levels of catalyst can be employed to improve the cure rate.